A basic idea of the cellular telephone system is to use the system's limited frequency band in such a way that despite limited frequencies it is possible to obtain the required capacity. This is achieved by forming cells. All the frequencies of the system are not available to the cell, but a certain group of frequencies only. The adjacent cell for its part can not use the frequencies of the frequency group of this cell in question, but these frequencies are available in such a cell only, which is located far enough from the cell in question. Signal strengths have hereby dropped sufficiently between cells using the same frequencies, whereby the interference of the same channel is also low enough and will not cause interferences in the radio channel. The allocation of frequencies on the described principle is called reuse of frequencies.
The matter presented above is essential in cellular planning intended to select cell size and system parameters, such as frequency allocation and cell capacity and number, so that such a continuous coverage is achieved economically which will support the required traffic density. Thus, factors to be taken into account in cellular planning are, among others, traffic density in different areas and the maximum transmission power and interference of mobiles.
The term `reuse factor` relating to the use of frequencies depends on the operator's cellular planning, and interference limitations also set up a limit for the reuse factor. The reuse factor has a decisive importance for the efficiency of the spectrum. The smaller the reuse factor, the more efficient is the use of the frequency spectrum. The reuse factor is determined by relative interference levels, C/I levels, wherein C is the level of the received carrier and I is the interference level. Each factor is affected by the used handover strategy, the power regulation of mobiles, discontinuous transmission DTX and frequency jumping.
In a completed network a separate frequency set is allocated for the cell, that is, a certain number of carriers at a certain frequency, and the reuse factor indicates how far from this cell the same frequencies are reused. Even though a certain number of carriers has been allocated for the cell, this does not always mean that all carriers are in use. The reuse factor is then "loose", whereby if the cell capacity must be increased, it may be increased by introducing more carriers from the cell's frequency set.
If the location and number of base stations are seen as constant, the frequency spectrum allocated for the system establishes an upper limit for the maximum capacity which the network may achieve. To illustrate this, reference is made to FIG. 1, which is a diagrammatic view of a geographical area covered by the cells in a cellular network. It is assumed for the sake of clarity, that the cells are of the same size and they may be presented by circles having the same radius, while base stations BTS shown by black dots are in the center of the cell and the distance between base stations is d. It is assumed that the reuse factor is 4, which means that four frequency sets are required: frequency set A, frequency set B, frequency set C and frequency set D. Thus, the same frequency set may reoccur so that a cell of one frequency set is between the cells of another frequency set, for example, in the manner shown in the figure. The carrier frequency is thus reused at distance D. Assuming that there are two carriers for each cell, that is, there are two carriers in each frequency set, whereby the total number of carriers would be 8, and assuming that a carrier requires a 200 kHz band as in a GSM system, the system shown in the figure as an example would require a frequency band of 1.6 MHz. This illustrates the problem concerning known networks that the network's frequency band sets an upper limit for the network's capacity. To increase the capacity the number of carriers must be increased, and this is possible only by making the frequency band bigger.
Interference can be taken into account as one criterion when the base station controller selects the frequency to be allocated for the connection. The base station may take this information into account when allocating those channels from free channels, which have as low a noise level as possible and when deciding on the cell's internal handover when it has been noticed that some channel of those which are used in traffic suffers from a higher uplink interference level than the ones which exist in any free channels. However, this can hardly be done in practice, because the operator wants the system to have as high a utilization rate as possible so that all channels would be in use. In practice, such a situation can hardly exist, especially in a network with interference limitation: if all channels would be in use in all cells, then the quality of connections would not be acceptable due to the rising interference level. For this reason, the network would be jammed on account of congestion, even if free channels might still be allocated. In such a situation the said interference level of uplink channels can be taken into account. In this way a kind of automatic channel planning between cells is achieved: the use of a certain channel in the first cell leads to an interference level of some magnitude in some other cell, thus preventing from taking into use any such channel which is interfered in this cell and which at the same time interferes with the first cell. This is basically a dynamic method of channel allocation, that is, if a cell is overloaded but cells wherein interfering channels are used are not, then the cell may use these channels temporarily.
According to the above presentation, a separate frequency set is allocated for each cell in a completed cellular network and the reuse factor indicates how far from this cell the same frequencies are reused. To achieve this, much frequency planning work has been needed. Frequency planning is fixed, that is, once frequencies have been allocated to the cells they are permanent. A drawback of fixed frequency planning is that it requires much work and it is not able to adapt itself to changes occurring in traffic volumes. When the network is complemented with new base stations, then a new frequency plan must be made. If a new base station is placed on the margin of the network, then the quantity of necessary work is reasonable, but if the base station or several base stations are located within the network between existing base stations so that their cell size is reduced, then frequency planning will require much work.
U.S. Pat. No. 5,212,831, Chuang and Sollenberger, presents a method and equipment for independent adaptive allocation of frequencies in FDMA and TDMA systems. According to the adaptive method, a frequency is allocated for each base station based on signal strength measurements performed by the base station. The measurements are done so that the base station turns off its own transmitters and listens to the downlink frequencies of other base stations and measures their signal powers. The received frequency with the lowest power is allocated temporarily as the downlink frequency of this base station. Each base station repeats this procedure independently and asynchronically in relation to other base stations. When all base stations have performed the procedure, then each has selected one downlink frequency. Then the same measurement cycle is performed again so many times that the downlink frequencies chosen by the base stations will no longer change or a predetermined number of iteration cycles has been performed.
In the method according to this invention there are at least two obvious drawbacks. Firstly, the base stations have to interrupt their transmission for the time of measurements, which is difficult because during that time and at least at that frequency connections with mobiles can not be kept up. This is harmful for network subscribers. Secondly, the strength of the downlink interference signal of other base stations is measured at the base station performing the measurements, whereby it is possible that the base station will not detect big interferences, but the serving mobiles will nevertheless suffer big interferences.
This second drawback is illustrated in FIG. 2. It shows a situation where base stations BS1 and BS2 are located in the landscape so that a signal between them will not proceed directly but strongly reflected. Such a situation will arise in urban conditions at crossings where buildings prevent the signal from proceeding straightly or in the country where high points in the landscape prevent a straight progress. Hereby, when using the method according to the US patent, BS1 would measure the transmission frequency of BS2 and would find that its interferences are low at this frequency. Correspondingly, BS2 would measure the frequency of BS1 and would find it small. The outcome would be that each base station could tune in on the same downlink frequency. It would result from this that when a car is at point A, it is in traffic connection with base station BTS1 at frequency f, whereby the connection would be of a good quality, but when the car has arrived at a crossing it would receive the frequency transmitted by base station BTS2, whereby it would experience the signal from base station BTS2 at the same frequency as a strong interference. Under these circumstances, at the crossing there would always be a strong interference in the mobile's reception, irrespective of which base station the mobile MS is in connection with.
An objective of the present invention is to bring about adaptive frequency planning without the drawbacks of known methods. The frequency planning must be able to work at the same time as the base station is in normal traffic use, and the planning must be able to take into account any real interferences which the mobile may experience.
The established objectives are achieved through the methods defined in the independent claims. The dependent claims present various ways of implementing the method.